1. Field of the Invention
This invention relates to the field of power control circuitry for an associated circuit. More particularly, this invention relates to power control circuitry for controlling connection of a voltage source to a switched power rail powering an associated circuit.
2. Description of the Prior Art
It is known to provide circuits (for example integrated circuits) including power rails connected via switch blocks to switched power rails (also referred to herein as virtual power rails). The logic blocks within the integrated circuit then draw their power from the virtual power rails. The switch blocks, which are typically high threshold voltage header and/or footer transistors, can be used to isolate the virtual power rail from the main power rail and accordingly isolate the logic blocks from the power supply. This is useful in reducing power consumption of the integrated circuit, e.g. by reducing the static leakage current therethrough. Such switches are sometimes referred to as power-gating switches.
It is also known that if a set of such switch blocks is turned on simultaneously, a significant supply current pulse may result, possibly large enough to cause state disruption in (or even damage to) components of the integrated circuit. This supply current pulse (or “surge”) results from the peak current required to recharge all the capacitive nodes of the integrated circuit. For this reason, the turn-on of the switch blocks may be delayed relative to each other to reduce this surge current.
Sequenced power control signalling is described in “Understanding and minimizing ground bounce during mode transition of power gating structures”, Kim et al., ISLPED 2003, Seoul, Korea, pp. 22-25.
Whilst the above approach is successful in avoiding a large supply current pulse when switching on, it suffers from the problem that switch blocks which are turned on early in a turn-on sequence suffer greater “stress” than switches later in the turn-on procedure, due to the fact that the initial supply current pulse, although reduced in magnitude in the integrated circuit as a whole, is nevertheless experienced most strongly by the switches used earliest in the turn-on procedure. The resulting heating effects and increased local current density caused by the initial supply current pulse lead to the degradation of those switch blocks by electromigration (the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms). As a result, during the lifetime of the integrated circuit, following repeated turn-on and turn-off cycles the increased stress experienced by some switches may cause them to fail significantly sooner than other less “stressed” switch blocks.